For a pump to function properly, pump packing materials must form a dynamic seal and, in addition, must be resistant to the effects of temperature, pressure, dynamic or system induced abrasive wear and chemical attack. Pumps used in industries such as the pulp and paper industry require packing materials that are particularly able to withstand the highly alkaline environments in which they are required to function. Unless the pump packing material is resistant to these highly caustic conditions, in which the pH often approaches or exceeds 14, the pump will be unable to perform and maintain its sealing function. If the pump packing material is degraded by these combined chemical and physical influences and thus loses its integrity, an acceptable seal will not be formed and the pump will not be able to function properly.
Pump packing materials designed to operate under the aforementioned conditions have been formed from asbestos fibers and yarns that had been impregnated or coated with a variety of tetrafluoroethylene (hereinafter TFE) coating and dispersion systems. The combination of asbestos and TFE produced a pump packing material that was highly resistant to both chemicals and adverse pump influences. These pump packing materials, moreover, were economical because of the low cost of the asbestos yarn used to make them. Because of their low cost and effectiveness, asbestos and TFE pump packing materials were commonly selected for use in pump applications in the pulp and paper industries and elsewhere where they would encounter high concentrations of caustics.
In recent years as possible health problems have been raised in connection with asbestos, packing manufacturers and users experimented with many alternative constructions in an effort to replace or eliminate asbestos fibers from packings. Other fibers, such as cotton, flax, hemp, rayon, acrylic, fiberglass, KEVLAR and the like, were employed in conjunction with various coatings and dispersions to form yarns that could replace the asbestos in pump packings. Some of these fibers proved equal to or better than asbestos in certain applications. However, even these asbestos substitutes were of limited utility in highly caustic environments such as those routinely encountered by pumps employed in the pulp and paper industry.
Because these fibers are not inherently caustic resistant, resistance to caustic attack is imparted to such fibers and to yarns formed from them by coating the fibers and/or yarn with a heavy TFE dispersion. Because of the superior resistance of TFE to caustic attack, non-asbestos fibers and yarns coated with TFE can withstand caustic induced degradation up to a pH of about 10 to 12. TFE itself has a virtually perfect resistance to caustic attack, and as long as the TFE coating remains in place on the fibrous material forming the pump packing so that the yarn is completely covered, the packing material will be able to function at the high pH ranges encountered in pulp and paper processing and in similarly caustic applications. Complete protection against caustic attack, however, is not provided by currently available methods of producing TFE fibers and yarns.
The fibers used to make pump packings are initially formed into yarns, which are then braided, woven or knitted to produce the packing material. Coatings may be applied either to the yarn prior to braiding or to the finished braid or to both. While TFE dispersions are capable of seeping into and coating the yarn and fiber bundles, even the best coating technology cannot assure a completely uniform, perfect coating. For example, TFE particles may be prevented from reaching the yarn where trapped air pockets or bubbles are formed so that no coating at all is applied in these areas. In other areas the coating may be very thin. When the pump packing is exposed to high caustic concentrations, uncoated or thinly coated areas of fiber become foci for chemical attack. Once the actual fiber is exposed to caustic chemicals, these chemicals will travel axially along the fiber because of the natural wicking action of the fiber so that the fiber will be subject to caustic degradation along its axial length. The result is a rapid loss of basic yarn integrity and a collapse of the sealing performance properties of the packing.
Further, the very nature of the function of pump packing material results in the material being subjected to wear and degradation. When a pump is in operation, the interface between the packing and the rotating or reciprocating pump shaft is continuously subjected to abrasion and higher temperatures than the rest of the packing. Consequently, this temperature and system-induced abrasive wear will eventually degrade even a thick TFE coating. Ultimately, the protective TFE coating will be removed, exposing the underlying fiber body. Once exposed, the fiber will then be rapidly destroyed by caustic attack with a concomitant loss of yarn integrity and collapse of the sealing properties of the packing. Because the packing material will always be subjected to wear while the pump is in operation, once the protective TFE coating is removed from a part of the packing body, the worn area may spread rapidly through the packing body. The result is that a greater percentage of the total material is exposed to caustic attack as the pump continues to operate.
The only inherently caustic resistant, "moderately" priced, non-asbestos fibers in existence that are suitable for use as dynamic pump packings are TFE filaments. TFE filaments may be combined with carbons and graphites, or these materials may be used singly to form a yarn that is naturally resistant to caustic chemical attack and degradation. They comprise the only solution presently available to the industry for producing pump packings for high caustic concentration, dynamic pump services. Although yarns made from the aforementioned materials provide excellent serviceability, it comes at a cost. The basic TFE yarns as provided by the yarn manufacturers prior to production of the finished packing are more costly than the old asbestos/TFE finished products were. The carbon and/or graphite packings also carry a premium price. The costs of manufacturing TFE, carbon or graphite yarns are higher than the costs of manufacturing other yarn types. Therefore, when the asbestos pump packing materials formerly used for caustic applications were replaced with a functionally equivalent TFE, carbon and/or graphite packing, it was at a substantial price disadvantage.
Moreover, additional problems and expense result from the need, given the available pump packing materials, to carry extensive inventories of different pump packings capable of functioning over only relatively limited pH ranges. For example, the relatively low cost general service pump packing materials are not useful at high pH ranges for the reasons discussed above. This necessitates stocking, in addition, expensive pump packing materials for high pH applications. A typical paper mill uses hundreds of caustic service pumps as well as large numbers of other pumps in its manufacturing operations. Consequently, the mill must presently maintain, generally at substantial added cost, a dual inventory of suitable pump packing materials and must employ and train maintenance personnel to handle and service properly this dual inventory.
The packing material disclosed in U.S. Pat. No. 4,559,862 to Case et al represents an attempt to solve the aforementioned problems of providing a low cost substitute for asbestos fiber pump packing materials that is inherently caustic resistant. The packing material described in this patent is formed from a yarn made of a combination of both inorganic fibers, such as glass, and organic fibers, such as sintered polytetrafluoroethylene or polypropylene, rayon and the like, and may include a polytetrafluoroethylene binder or coating. While a yarn having such a composition presents some advantages, it also suffers from obvious disadvantages. If polytetrafluoroethylene is used as a coating or binder, the problems resulting from incomplete or uneven coating coverage discussed above are not overcome. If polytetrafluoroethylene is used in the form of fibers, the cost of the yarn becomes high. Consequently, the combination of inorganic glass-type fibers and organic fibers coated with polytetrafluoroethylene described in this patent does not provide the resistance to degradation at high caustic concentrations required by many pump applications.
The prior art, therefore, has failed to disclose a pump or valve packing material for use at high caustic concentrations that is characterized by both low cost and a long useful service life. The prior art has further failed to provide a method whereby low cost non-asbestos fibers may be rendered caustic resistant and then used to form such a caustic resistant pump packing material.